Central or decentral heat-recovery ventilation: which one for a house

How each works

Both are balanced mechanical ventilation with heat recovery: stale air goes out, fresh air comes in, and a heat exchanger passes the heat from one to the other without mixing them. The difference is where the exchanger sits and how the air reaches the rooms.

Central heat recovery ventilation: how it works

A single unit (an HRV, or an ERV with moisture recovery) sits in a plant room, in the loft or above a suspended ceiling. Two ducts link it to the outside through two holes in the facade or the roof: fresh air and exhaust air. Inside, one duct network brings fresh air, filtered and warmed by the exchanger, to the living room, bedrooms and study, and another takes it out of the kitchen, bathrooms and store room. In between, the air moves through the hall, under the doors. Each room gets what it needs, set at the valves when the system is commissioned.

Wall-mounted heat recovery unit: how it works

A decentral unit sits in a hole through the outside wall of the room it ventilates, with no ducts. There are two kinds. A two-fan unit has a continuous exchanger inside (counter-flow, sometimes with a membrane that also recovers moisture): air comes in and goes out at the same time, like a small central unit. All five decentral units certified by the Passive House Institute on 31 October 2026 are of this kind.

Push-pull heat recovery units in pairs: how they work

An alternating (push-pull) unit has one reversible fan and a ceramic block. For about a minute it blows warm room air out and the ceramic warms up; then the fan reverses and cold outdoor air comes in through the warm ceramic, cooling it. To keep the flow balanced the units work in pairs: while one blows out, the other blows in, even when they sit in different rooms. They usually have axial fans, which a small pressure difference can stop.

Air flows and distribution

The air flow is worked out the same way for both, with the Passive House Institute method of the ventilation calculator: 30 m³/h per person, at least 0.30 air changes an hour, rounded up to 10 m³/h. For the 150 m² house that is 120 m³/h (0.32 h⁻¹). What differs is how the air reaches the rooms.

Room by room with central ventilation

Fresh air is shared between the living rooms (the living room 45 m³/h, each bedroom 25 m³/h), passes through the halls and leaves from the wet rooms (the kitchen 40 m³/h, each bathroom 25 m³/h, the plant room and store room 15 m³/h each). The same air serves twice: first in the bedroom, then in the bathroom. That is why 120 m³/h is enough for the whole house.

How many wall-mounted heat recovery units do I need

A wall unit ventilates only the room it sits in, and takes out as much as it brings in. The page’s rule: every living room gets its supply flow, the kitchen, bathrooms and WC their extract flow, with at most 25 m³/h per unit (the continuous range of the certified unit used in the calculation). For the 150 m² house: living room: 2 × 22.5 m³/h, 3 bedrooms: 1 × 25 m³/h each, kitchen: 2 × 20 m³/h, 2 bathrooms: 1 × 25 m³/h each, that is 9 two-fan units or, rounded to pairs, 10 alternating units. Between them they exchange 210 m³/h with the outside, 90 m³/h more than the central unit, because air no longer passes from one room to the next. The plant room and the store room get no unit (30 m³/h of extract missing).

Heat recovery ventilation without ducts: transfer air

Without ducts there is no controlled path through the house. Each unit works on its own facade, and the wind and the temperature difference between low and high (the stack effect) press differently on each wall; closed doors separate the rooms. The EwWalt project (RWTH Aachen, HLK Stuttgart, ITG Dresden) showed that alternating units placed on different facades, “across corners”, see large pressure differences between them (REHVA Journal 4/2019). Bathrooms without a window or an outside wall cannot take a unit and stay on a simple extract fan with no recovery.

Heat recovery in practice

The Passive House Institute certifies central units with 79–94 % heat recovery (the ventilation calculator’s list) and decentral units with 78–86 % (PHI database). Certification figures are measured in a laboratory, with balanced flows and no wind.

What is lost in use: wind, imbalance, stack effect

A wall unit has no ductwork to shield it from the pressure on the facade. Filis, Kolarik and Smith (Energy and Buildings, 2021) simulated a full year: wind alone reduces heat recovery by up to 8 % in units with centrifugal fans and up to 22 % in units with axial fans; with the stack effect, up to 20 % and 52 %. The same paper cites field measurements on 60 room units (Merzkirch et al.): imbalances of up to 60 % from wind or stack effect and average heat recovery about 8–22 % below the manufacturers’ values, but also fans using less than 0.25 Wh/m³, about half of the central units measured. In alternating units, the effective mean flow was about 85 % of the flow in continuous operation, and in strong wind at low speed it can fall to zero (EwWalt). We could not read the Merzkirch field study itself; its figures are as cited by Filis et al.

Heat lost a year, Bucharest

For the comparison: the central Zehnder ComfoAir Q350 HRV with 91 %, at 120 m³/h; wall units with 85 % (Zehnder ComfoAir 70, the best efficiency ratio on the PHI list) at 210 m³/h, once as in the laboratory and once with 10 points less, the page’s assumption for real use, within the range of the studies above. With 74.3 kKh to 23 °C, central ventilation without recovery would lose 2,942 kWh a year; with it, 265 kWh. The wall units lose 772 kWh in laboratory conditions and 1,287 kWh in use: 2.9–4.9 times as much, mainly because they move 210 m³/h instead of 120 m³/h.

Passive House certified wall units

All single-room decentral units in the Passive House Institute database, read on 31 October 2026, with their certificate values. No prices; we do not recommend a particular manufacturer.

Noise

Wall-mounted heat recovery unit noise

The Passive House Institute asks for at most 25 dB(A) in living rooms and 35 dB(A) in the plant room. With central ventilation the fans sit in the plant room and the duct silencers keep the noise away from the bedrooms. With a wall unit the fan is in the room. The certified units meet the limit only in their continuous range: for example, at 50 m³/h one unit gives 25 dB(A) with its best cover and 34 dB(A) with the standard one, and another passes 25 dB(A) above 30 m³/h (the certificates in the table). That is why the page’s calculation does not load a unit with more than 25 m³/h. For uncertified units there are no comparable measurements. One more thing: a Ø 160–200 mm hole through the wall with a unit in it lets in more outdoor noise than a solid wall.

Filters and maintenance

Filters: how many and which class

The PHI certificates ask for a fine ePM1 50 % (F7) filter on the outdoor air, which stops pollen and much of the fine dust, and at least ISO Coarse 60 % (G4) on the extract air, which protects the exchanger from dust. A central system has two filters in one place; the 150 m² house with wall units has 9 units, each with its own filters, in different rooms, some under the ceiling. Many uncertified units have coarse filters only.

Time and money a year

The page’s assumption (indicative prices, 31 October 2026): filters are changed twice a year; a set for the central unit costs €47 and takes about 15 minutes, a set for a wall unit €11 and about 10 minutes, plus washing the ceramic block of alternating units. A year: 2 changes, €93 and half an hour for the central system; 18 changes, €202 and about 3 hours with wall units. Central ductwork is checked and, if needed, cleaned every few years.

Frost and humidity

Heat recovery unit in winter: frost

In cold weather the moisture in the extract air can freeze in the exchanger. The central unit has an electric pre-heater that starts below −1.5 °C (the PHI switch-on point): in Bucharest, 482 hours a year and 67 kWh of electricity, and the flows stay balanced. Certified wall units generally have no pre-heater: they protect themselves by unbalancing the flows or stopping the supply air. The certificates say how: one reduces the supply air from −8.2 °C and stops it at about −13 °C, another reduces the flow from −9 °C, another unbalances from −9.5 °C, and one has an external 600 W pre-heater from −4 °C. Bucharest has 64 hours a year below −8 °C and 17 below −13 °C, and its design temperature is −15 °C (SR 1907-1). In those hours the house gets less fresh air, or colder air, and the missing air comes in through leaks.

Humidity and condensation

Moisture recovery (ERV) exists for both: four of the five certified wall units also recover moisture, 54–64 %. PHI writes on the same certificates that in cool temperate climates moisture recovery should only be used where the house produces little moisture, and that above 60 % humidity-controlled flow is needed. What that means for a house in Romania is on the page on relative and absolute humidity and in the HRV or ERV lesson of the ventilation calculator. A wall unit drains what condensate it has to the outside through the facade cover (as one of the certificates says).

Holes in the facade, airtightness, thermal bridges

How many holes through the insulated wall

The central option has 2 penetrations of the envelope, usually together next to the unit: fresh air and exhaust. Wall units need 10 (or 9) core-drilled holes of Ø 160–200 mm through the insulated wall, one per unit, on every facade. Each has to be sealed to the airtight layer inside and to the insulation outside, and each is a small point thermal bridge.

Airtightness of a Passive House

A Passive House has n₅₀ ≤ 0.6 h⁻¹ in the pressure test: all air that passes through a leaky hole does so without heat recovery. Each extra penetration is one more detail to build well and one more chance that it is not. We give no figure for how much one hole leaks; only that ten sealing details spread over the facades are harder to make and check than two. On thermal bridges and mould: the page on thermal bridges and mould.

Cost

The central unit with ducts and installation for the 150 m² house: €2,620 + €2,240 + €1,680 = €6,540 (the ventilation calculator’s figures; market guides give €4,670–6,540 for a new house). One installed wall unit costs €560–1,500 (Onninen guide); 10 units: €5,610–14,950. A year: the lost heat at €0.071/kWh (gas at €0.065/kWh, a 92 % boiler), electricity at €0.234/kWh, and the filters.

Side by side

When each makes sense

Central or decentral ventilation: when to choose which

Central for new houses, deep retrofits, Passive House and nZEB: flows are designed room by room, air moves through the house from living rooms to wet rooms, recovery stays as measured, and filters, noise and frost are dealt with in one place. Decentral for one or two rooms of an existing home, for a flat where no ducts can go, or as a first step before a larger retrofit. There a certified wall unit within its continuous range is far better than an open window in winter; PHI itself, in the description of the list, calls them “particularly suitable for renovation projects”, and for small homes.

What Semper Supra recommends

We design central systems and do not install wall units; the reader should know that when reading the recommendation. For a whole house we recommend the central option for the reasons on this page: for the 150 m² house it loses 2.9–4.9 times less heat, has 2 filter changes a year instead of 18, 2 holes in the facade instead of 10, and keeps the flows balanced at −15 °C, at a cost close to the middle of the decentral range. If your home is one where ducts do not fit, a certified wall unit is a good choice, not a second-rate one. Flows and the unit for your house: the ventilation calculator; the design: Semper Supra’s services.